Comparative biochemical investigations of schistosomes and filariae and their mammalian and invertebrate hosts have revealed that certain differences in the parasites' nucleic acid metabolism are potentially exploitable for chemotherapy by appropriate metabolites. Moreover, these investigations are providing some insight into certain aspects of host-parasite interrelationships which could lead to novel approaches to vector control. Of high priority will be continued investigations of the properties of xanthine dehydrogenase (XDH) which we have established as being a constitutive enzyme in adult Aedes aegypti and of our earlier finding that certain properties of this enzyme are altered when mosquitoes take a blood meal containing Brugia pahangi microfilariae. Under these circumstances the expected increased purine-catabolizing activity of mosquito XDH (found 12 and 24 hours after mosquitoes ingest a normal blood meal) was not observed. Studies will continue concerning the mechanisms of action of the schistosomicidal purine nucleoside analog, tubercidin (7-deazaadenosine). There is evidence that in addition to effects upon endogenous ATP levels in the worms, tubercidin also adversely affects their production of cyclic AMP but not cyclic GMP. These intriguing new findings form the basis for investigations of hitherto unrevealed facets of schistosomal biochemistry and physiology. BIBLIOGRAPHIC REFERENCES: Jaffe, J.J., Doremus, H.M., Dunsford, H.A., and Meymarian, E. 1975. Long-term efficacy of tubercidin against schistosomiasis japonica and mansoni in primates. Am. J. Trop. Med. Hyg. 24: 289-297. Jaffe, J.J., Doremus, H.M., Dunsford, H.A., and Meymarian, E. 1975. Treatment of schistosomiasis mansoni and japonica in baboons with tubercidin given by direct intravenous injection. Am. J. Trop. Med. Hyg. 24, in the press.